The great thing about the internet is that you can find offline things to do from it. If you want to leave the house and get involved locally in an activity, it is a hard step away. You can search or look at some sites that create groups or post activities to browse exactly what it is you are looking for. If something is not in your city that you want to do, you can create something and post it yourself!
The first thing would be to decide what exactly it is you are looking for. If you want to play a sport, join an educational group, or see live music. If what you are looking for does not exist, then think about what it is you want and prepare a posting to use on one of the sites talked about in further detail below.
Certain niches have different sites to look at. However, if it is something general you are looking for, a few sites will be a good place to start. Meetup is a good resource, but depending on where you live there might not be many groups available.
Other places to look can include online classifieds sites. Through these you can post anonymous information if you want, or find other likeminded hobbyists in your area.
The great thing about both of these, is that they are location specific as opposed to theme specific. Looking for something of a specific theme or topic you will definitely find through a search engine, but we would like to know how many people interested in this topic live close to where we are. This is where I find classifieds sites to be the best. There are a few classifieds sites with which you can search such as Craigslist. Or you can choose to search new sites such as classivox.
Do not forget, if you can not find what you are looking for then the best thing to do is create a posting yourself. Posting on some sites will be much faster than others. For example meetup you have to create an account, and learn how the site works. This can be time consuming and you may not have the time to do this. Other options like the classifieds sites I mentioned above are a much faster way to communicate.
Whichever method you choose to use in your search for offline activities, I hope you have fun and meet new people in your adventure!